1. Field of the Invention
The present invention relates to a super twisted nematic (to be referred to as STN hereinafter) liquid crystal display device and a liquid crystal display device for enlarging a displayed image and allowing observation of the image.
2. Description of the Related Art
As a liquid crystal display device for displaying an image by using a liquid crystal cell, a display device using an STN liquid crystal cell is available. In this device, the multiplexing drive performance is improved by setting the twist angle of the liquid crystal molecules to be as large as 180.degree. to 270.degree..
In the liquid crystal display device using such an STN liquid crystal cell, one of a pair of polarizing plates respectively arranged on the incidence side and emergence side of the liquid crystal cell is arranged such that the direction of its polarization axis (axis of transmission or absorption) has a certain deviation angle (35.degree. to 50.degree.) with respect to the direction of liquid crystal alignment on the surface of a liquid crystal cell substrate opposed to the polarizing plate. With this arrangement, linearly polarized light incident through the polarizing plate on the incidence side is converted into an elliptically polarized light by the birefringence characteristic of the liquid crystal layer of the liquid crystal cell, and the transmission of light is controlled by the polarizing plate on the emergence side, thereby displaying a corresponding image. Since the refractive index anisotropy of the liquid crystal layer varies with light wavelength, the transmittance of light in a specific wavelength range is increased, resulting in coloring in display.
For this reason, in a liquid crystal display device using the above-described STN liquid crystal cell, in order to prevent coloring in display, an STN compensation liquid crystal cell in which liquid crystal molecules are aligned to be twisted in a direction opposite to the liquid crystal twist direction of the display liquid crystal cell having display electrodes is arranged between the STN display liquid crystal cell and one of a pair of polarizing plates respectively arranged on the incidence and emergence sides of the display liquid crystal cell.
In this liquid crystal display device, retardation on the liquid crystal layer of the display liquid crystal cell and retardation on the liquid crystal layer of the compensation liquid crystal cell are caused to cancel each other. More specifically, the liquid crystal molecules of the display and compensation liquid crystal cells are respectively aligned to be twisted through the same angle in the opposite directions, and the liquid crystal alignment directions of the display and compensation liquid crystal cells are set to be perpendicular to each other on the respective adjacent substrate surfaces. With this arrangement, in an initial state wherein no voltage is applied across opposing electrodes of the display liquid crystal cell, light transmitted through the display liquid crystal cell and the compensation liquid crystal cell substantially becomes linearly polarized light and is incident on the emergence side polarizing plate, thus preventing leakage of light in a specific wavelength range and coloring in display.
In a known conventional liquid crystal display device of this type, the same liquid crystal is used for a display liquid crystal cell and a compensation liquid crystal cell, and the liquid crystal layer thicknesses (cell gaps) d of the two cells are set to be equal to each other so that the values of .DELTA.n.multidot.d (the product of refractive index anisotropy .DELTA.n of a liquid crystal and the liquid crystal layer thickness d) of the two cells coincide with each other. As the liquid crystal sealed in the two cells, a liquid crystal having a refractive index .DELTA.n of 0.6 to 0.7 is used to increase the range of angles from the normal direction in which an image formed by the liquid crystal cell can be clearly seen (to be referred to as a viewing angle characteristic hereinafter). In addition, the value of .DELTA.n.multidot.d of each cell is set to be 0.14 or less.
Although the conventional liquid crystal display device has a good viewing angle characteristic and high contrast in display, the response time (the operation time of the liquid crystal of the display liquid crystal cell) to a driving voltage is as long as about 150 msec. That is, the conventional device has a low response speed. Therefore, if a motion image such as a television image is displayed, the displayed image is seen as if it is flowing.